Cytokeratin positivity in fine-needle aspirates of melanomas and sarcomas

Cytokeratin (CK) positivity has been considered a specific marker for epithelial differentiation in cytologic specimens. After observing CK reactivity in fine-needle aspirate (FNA) specimens of melanoma and sarcoma, a retrospective study of melanomas and sarcomas was undertaken to investigate the frequency of anomalous CK staining in these neoplasms. Cell block sections and/or restained smears from 36 melanomas and sarcomas were retrospectively stained for CK. An appropriate internal positive control (HMB-45), S100 protein, or vimentin) was also used to insure antigen preservation. Of the smears from 19 melanomas, five revealed focal strong CK positivity of neoplastic cells, two cases showed faint or equivocal staining, 11 cases were negative for CK, and one could not be interpreted due to inadequate controls. Of the smears from 14 sarcomas, two showed positivity for CK (one fibrosarcoma and one condrosarcoma), 11 were negative, and one had inadequate controls. The number of CK positive cells was less than that observed with the appropriate internal positive control antibodies. Destained Papanicolaou smears were superior to Diff-Quik smears for retrospective immunocytochemical stains. Cell block sections from four of the melanomas and one sarcoma demonstrated no aberrant staining. Since cytokeratin positivity occasionally is seen in nonepithelial neoplasms, its presence alone cannot be used to make a definitive diagnosis of carcinoma. Therefore, a panel of immunocytochemical stains should be utilized in diagnosis of FNA specimens.